We are studying the function of host extracellular matrix components in the pathogenesis of Candida albicans. Using in vitro assays to quantify adhesion of C. albicans to immobilized fibronectin and its proteolytic and recombinant fragments and to evaluate binding of soluble fibronectin to C. albicans in suspension, we found that interactions of fibronectin with this organism are not mediated by the Arg-Gly-Asp integrin recognition sequence of fibronectin. High affinity binding, observed following grown in complex medium, is primarily mediated by the collagen-binding domain of fibronectin. A 30 kDa fragment of fibronectin containing the collagen binding domain is as active as intact fibronectin for binding to C. albicans. Expression of fibronectin receptors is tightly regulated by growth conditions. C. albicans grown in defined media do not bind fibronectin. We identified hemoglobin as a highly specific activator of receptor expression, which reversible induces fibronectin binding when added to defined growth medium. This binding is of lower affinity and is mediated by the cell-binding domain of fibronectin. Hemoglobin-inducible binding was observed in all clinical isolates of C. albicans and in other members of the Candida genus. This activation may play an important role in pathogenesis, since only pathogenic strains of C. albicans express hemolytic activity. Inhibitors of this activation process may decrease the pathogenicity of C. albicans. We have identified ESTs for several genes whose expression is induced by hemoglobin and a 55 kDa receptor that mediates inducible binding to fibronectin.Fibronectin (FN) receptor expression was induced by ferric (Hb+Met, Hb+CN), ferrous (HbCO, HbO2), and cobalt-protoporphyrin derivatives of Hb, whereas globin was inactive. The Hb derivatives all showed saturable, dose-dependent kinetics of FN receptor induction suggesting that Hb may be acting as a receptor ligand. Soluble Hb bound saturably to a low affinity binding site (Kd = 1.1 .2 I 10 -6 M) on C. albicans blastospores. However, uptake kinetics of 55FeHb revealed that heme or iron transport into the cell is not requisite for induction, since internalization of 55Fe from Hb did not occur until after induction of FN binding. The serum Hb-binding protein, haptoglobin, specifically abrogated this response, indicating that protein structure rather than the heme ligand or iron is necessary for induction of this signaling pathway. These data demonstrate that C. albicans recognizes Hb through multivalent low affinity interactions, and this may be a host environmental cue that triggers extracellular matrix receptor expression at a septic site.